1. Field of the Invention
The present invention relates to an image forming apparatus capable of controlling the charge of toner and a method thereof. More particularly, the present invention relates to an image forming apparatus and method for determining the charge of a toner from a development current with respect to a development voltage supplied to a developing roller and control the charge of the toner based thereon.
2. Description of the Related Art
Generally, an image forming apparatus adopting an electrophotographic method, such as a laser beam printer, which is also simply referred to as a laser printer, forms an image by forming an electrostatic latent image by scanning a laser beam to an organic photoconductor and attaching toner to the electrostatic latent image. Herein, the toner is typically charged with a negative (−) voltage to attach the toner to the organic photoconductor. The quality of the image formed in the image forming apparatus depends on the quantity of the toner attached to the organic photoconductor based on the charge of the toner.
FIG. 1 is a schematic diagram illustrating a conventional electrophotographic image forming apparatus.
The conventional image forming apparatus comprises a toner supplying roller 10, a developing roller 30, a blade 36, a charging roller 60, an organic photoconductor 50, and a fixing roller 70. The toner supplying roller 10 is charged with a high negative voltage, e.g., −500V, by a first power source 20 and provides toner to the developing roller 30. The developing roller 30 is charged with a high negative voltage, e.g., −300V, by a second power source 35, receives the toner attached to the toner supplying roller 10 by a potential difference from the toner supplying roller 10, and supplies the toner to an organic photoconductor (OPC) 50. The blade 36 maintains the uniform thickness of the toner attached to the developing roller 30. The charging roller 60 charges the organic photoconductor 50 with a high negative voltage, e.g., −700V, by a third power source 65. The organic photoconductor 50, which is charged by the charging roller 60, forms an electrostatic latent image in the part scanned by a laser scanning unit (LSU) 80 and attaches the toner to the electrostatic latent image. The fixing roller 70 fixes the toner attached to the electrostatic latent image which is formed in the organic photoconductor 50 on paper P. In the image forming apparatus with the above-described structure, the charge of the toner depends on a surrounding environment of the image forming apparatus, such as, a temperature and humidity. If the charge of the toner is increased, the quantity of the toner attached to the developing roller 30 is increased more than necessary and thus ghost images may be generated in the image printed on the paper P by the organic photoconductor 50 and the fixing roller 70, or the concentration of the toner on the print is increased more than necessary. If the toner has a lower charge, the toner layer attached to the developing roller 30 becomes so thin that it becomes a film on the surface of the developing film 30 or on the surface of the organic photoconductor 50. In this case, the quantity of the toner supplied to the organic photoconductor 50 is insufficient and the image printed on the paper P is indistinct or streaks are generated on the image.
FIG. 2 illustrates an example of the force acting on the toner between the organic photoconductor 50 and the developing roller 30.
The toner T is supplied to the organic photoconductor 50 by a potential difference between the developing roller 30 which is charged with about −300V and the organic photoconductor 50 which is charged with about −700V. Herein, a force Fd in which the toner T moves towards the organic photoconductor 50 is in proportion to the charge q of the toner and an electric field Ed between the developing roller 30 and the organic photoconductor 50. The force Fd can be expressed as an equation 1 below.Fd=q×Ed  Eq. 1
Meanwhile, a force Fr in which the toner T is attached to the developing roller 30 is a summation of a Van der Wall's force and an image force. The Van der Wall's force Fm is expressed as an equation 2 below.
                    Fm        =                              q            2                                4            ⁢            π            ×            ɛ            ⁢                                                  ⁢                          r              2                                                          Eq        .                                  ⁢        2            
wherein q denotes the charge of the toner, and εr denotes a dielectric rate of air.
Based on the equations 1 and 2, the charge of the toner T and the forces Fd and Fr according to the charge can be expressed as a graph shown in FIG. 3.
It can be seen from FIG. 3 that the toner T attached to the developing roller 30 is supplied to the organic photoconductor 50 in appropriate amounts at a charge q′ where the force Fd having a shape of a one-dimensional function and the force Fr having a shape of a two-dimensional function has the largest difference. The charge q′ has a different value according to electrophotographic image forming apparatuses. An electrophotographic image forming apparatuses produced by the Samsung Electronics company has a value of 12 to 20 uC/g. As shown in FIG. 3, if the toner T is charged less than 12 uC/g, or if the charge of the toner T is charged more than 20 uC/g, the quantity of the toner T attached to the developing roller 30 is decreased or increased, respectively, to an abnormal level and the toner T is not attached to the developing roller 30 uniformly and mechanical friction with the blade 36 which limits the thickness of the toner T attached to the developing roller 30 is increased. The increase in the mechanical friction between the blade 36 and the toner T can strip off the additives added to the toner T, such as SiO2 and TiO2. The additives are added to the toner T in the form of surrounding toner T particles to thereby prevent toner particles from being combined with each other and from being fused onto the organic photoconductor 50. If the additives are removed or damaged, the toner T can be fused onto the organic photoconductor 50 in the form of a film or degrades the quality of the image formed in the image forming apparatus.